1. Field of the Invention
The present invention relates to a water-absorbing laminate and a process for producing the same. More specifically, it relates to a water-absorbing laminate which can be used as a sheet for a cover culture (or shade culture) or the like and a process for producing the same.
2. Description of the Related Art
Plastic films which have been conventionally used for a cover culture such as a plastics greenhouse, a plastics tunnel, or a greenhouse culture have heat-retaining properties at night, but are liable to create excessive moisture and thereby cause waterdrops to drop. Excessive moisture and the subsequent dropping of waterdrops not only deteriorate the working environment of a plastics house or greenhouse but also cause the plants being cultured to become diseased and a reduction in the yield and quality of the plants, with the result that the gains become poor.
On the other hand, when a nonwoven fabric having a relatively thin thickness is used, as the covering material, excessive moisture can be avoided because of the gas permeability, but satisfactory heat-retaining properties cannot be obtained and the dropping of waterdrops cannot be eliminated to a satisfactory degree. Therefore, it is considered that laminates composed of plastic films and fibrous sheets such as nonwoven fabrics are utilized. However, the use of conventional plastic films for such laminates does not lead to the avoidance of the formation of excessive moisture.
In view of the above-mentioned situations, there is proposed a laminate comprising a film of polyvinyl alcohol (hereinafter referred to as PVA) having a high hydrophilicity and hygroscopicity and excellent weathering resistance and a nonwoven fabric which are integrally laminated to each other (see Japanese Unexamined Patent Publication (Kokai) No. 56-67251). It has been established that the laminate described in the above-mentioned publication is extremely effective for avoiding the formation of excessive moisture and for preventing the dropping of waterdrops while retaining the heat-retaining properties, when it is used for a cover culture such as a plastics greenhouse, a plastics tunnel or a greenhouse culture. However, especially when the inside of a heated plastics house or a greenhouse is lined with the above-mentioned laminate, waterdrops on the ceiling, which is made of a plastics film for a plastics greenhouse or of a glass sheet, an acrylic sheet, or a polyester sheet for a greenhouse drop at a higher rate than the rate of the water-absorbing diffusion of the upper PVA film of the laminate during winter nights, especially before daybreak. As a result, the surface of the PVA film is saturated with water. In a case where puddles are locally formed on the PVA film surface, the water temperature rises due to the subsequent sunlight irradiation in the morning causing a phenomenon in which an extremely small amount of PVA is dissolved in the water. If such a phenomenon is repeated over a long period of use, the laminate becomes thinner and the so-called strength is lowered.
In order to improve the mechanical strength of the above-mentioned laminate and the lining workability in a plastics house or a greenhouse, it is necessary to impart flexibility to the laminate. If hydrophilic flexibilizers such as glycerol and glycols are added to the PVA film of the laminate to impart flexibility thereto, the hydrophilic flexibilizers are dissolved in the water and the aqueous solution drops on the plants under culture. Since this aqueous solution may damage the plants, the addition of such hydrophilic flexibilizers is limited.
The present inventors discovered that if these disadvantages could be overcome, further excellent agricultural materials can be provided.